Position sensors are used in various applications to determine the position of objects relative to surfaces. For example, some scanning devices (sometimes referred to herein simply as scanners) use position sensors to determine the position of the scanner relative to a document that is being scanned. In another example, some printers use position sensors to determine the position of paper being printed on relative to the device within the printer that actually prints on the paper, e.g., a print carriage.
A scanner is a device that converts an image of an object, e.g., a page of text, to machine-readable image data (sometimes referred to herein simply as image data), referred to herein simply as image data. Some scanners convert a narrow scan line portion of the image of the object to image data. In order to generate image data representative of the image of the entire object, the scanner is moved relative to the object. As the scanner is moved relative to the object, the scanner generates image data representative of a cumulation of sequential scan line portions of the image of the object. The image of the object is, thus, represented as this cumulation of sequential scan line portions of the object, similar to a video display of the object.
The image data is typically processed by, and stored in, a computer, which may be used to replicate or modify the image of the object. For example, the image data may be transmitted via a data line to another computer or facsimile machine, that replicates the image of the object. In the case where the object is a page of text, the image of the text may be input into the computer and edited by a word processing program.
It is critical that the computer knows where, in relation to the surface of the object, the scan lines were generated in order to properly process the image data. Determining the positions on the object from where scan lines were generated may be achieved by determining the position or velocity of the scanner relative to the object as the image data is being generated. The image data representing the scan lines may be electronically tagged with the locations of the scan lines relative to the surface or relative to each other. During processing, the computer may then properly place the scan lines relative to each other to replicate the image of the object.
The locations of the scan lines relative to the surface of the object are used by the computer to replicate the image of the object. One aspect to replicating the image of the object is determining the size of the object in the dimension defined by the movement of the scanner relative to the surface of the object. For example, if the scanner is generating 1000 scan lines per second and the relative velocity between the scanner and the object is established at one inch per second, the computer will process the image data based on each scan line representing one one-thousandth of an inch of the image of the object. If, however, the relative velocity decreases and the decreased velocity is not accurately conveyed to the computer, the computer will continue to process the image data as though each scan line represents one one-thousandth of an inch of the image of the object. This will result in the image of the object represented by the image data being compressed, which is not an accurate representation of the object. If, on the other hand, the relative velocity is increased and the increased velocity is not accurately conveyed to the computer, the image of the object represented by the image data will represent an expanded image of the object, which is also not accurate.
Some scanners use roller mechanisms to generate information pertaining to the position of the scanner relative to the surface of the object being scanned. This position information is processed to determine where, in relation to the surface of the object, the scan lines were generated. The roller mechanism contacts the object and rotates as the scanner is moved relative to the object. The scanner measures the rotation of the roller mechanism to determine the position of the scanner relative to the object. Roller mechanisms, however, do not provide a direct measurement of the position of the scanner relative to the object. Instead, the position measurement is derived from the rotation of the roller mechanism, which may add inaccuracies to the position measurement. Furthermore, the roller mechanism relies on friction between the roller mechanism and the object in order to maintain rotation. If the friction is decreased for any reason, the roller mechanism may slip rather than rotate, which will cause the position measurement and, accordingly, the image represented by the image data, to be inaccurate.
Other scanners use optics to determine the position of the scanner relative to the object. As an example, optical detectors attached to the scanner continually image small two-dimensional areas of the object and transmit image data representative of these areas to a computer. The computer identifies distinct features of the object that are located in these imaged areas and stores the locations of these distinct features relative to the optical detectors. These distinct features may, as an example, be variations in the surface of paper caused by pulp material used in the manufacture of paper. As the scanner is moved relative to the object, these distinct features move relative to the optical detectors. The computer identifies the direction and amount of movement of these distinct features relative to the optical detectors to determine the position, direction of movement, and velocity of the scanner relative to the object.
Scanners using these optical detectors, however, require extensive processing capabilities in order to identify distinct features on the object and to determine the movement of the distinct features relative to the optical detectors. These extensive processing capabilities increase the cost and complexity of the scanner. Another problem with these optical detectors is that the areas on the object that they image are required to be illuminated in order to detect distinct features on the object. This addition illumination increases the power requirement of the scanner beyond the requirements typically required to scan the object. This additional power requirement is inherently detrimental to hand-held scanners that rely on portable power supplies. Furthermore, the optical detectors require additional optical components, e.g., lenses, to be used in the scanner, which increases the cost and complexity of the scanner.
Therefore, a need exists for a position sensor that will directly and accurately measure the position or velocity of a first object relative to a surface of a second object and that requires minimal power and processing requirements.